The invention relates to the field of optical data storage and retrieval and more particularly to scanning optical system used for such storage and retrieval.
The invention relates to an optical device for scanning an information layer in an optical record carrier having at least two information layers, said device comprising a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot on the desired information layer, a composite diffraction element arranged between the radiation source and the objective system and comprising two contiguous gratings for diffracting a radiation beam coming from the desired information layer to a composite radiation-sensitive detection system and for splitting said beam into a first and a second sub-beam constituting a first and a second detection spot in the plane of the detection system, the detection system comprising separate detection elements for the detection spots, with two detection elements for at least one of the detection spots.
A device of this type is known from, for example, U.S. Pat. No. 4,665,310 for reading an optical record carrier having a single information layer. In this device, the diffraction element fulfils two functions for which otherwise two separate elements would be required. First, this element ensures that the radiation beam which is reflected and modulated by the information layer and passes through the objective system follows a different optical path than the beam emitted by the radiation source, so that a radiation-sensitive detection system can be placed in the path of the modulated radiation beam. Furthermore, the composite diffraction element splits the reflected beam into two sub-beams by means of which a focus error signal is obtained, i.e. a signal comprising information about the magnitude and the direction of a deviation between the focal plane of the objective system and the information layer. To this end, the detection system of the known device comprises four detection elements which constitute two pairs. Each of the two sub-beams is applied to a separate pair of detection elements. A sub-beam is converged to a radiation spot, hereinafter referred to as detection spot, on the associated pair of detection elements. When the scanning beam is focused on the information layer, the detection spot is minimal and the center of this spot is situated symmetrically with respect to the separating strip in the relevant pair of detection elements. The elements of the relevant pair then receive an equal amount of radiation from the relevant sub-beam. If a deviation occurs between the focus of the scanning beam and the information plane, the detection spot becomes asymmetrically larger and the detection elements of the relevant pair receive different amounts of radiation from the relevant sub-beam. The difference between the output signals of the elements of the same pair is thus indicative of the extent to which the scanning beam is focused on the information plane. In the known device, the boundary line between the two gratings is a single straight line which is perpendicular to the optical axis of the objective system and, for example, perpendicular to the line of connection between the centers of the two pairs of detection elements.
To increase the storage capacity of an optical record carrier, it has already been proposed to incorporate two or more information layers instead of one in this record carrier. To be able to read all information layers, all of these layers, with the exception of the last of the stack, should be partially transparent. When a given information layer is being read, the scanning beam must be focused sharply, and remain focused sharply, on this layer. When another layer is being read, the focus of the scanning beam must be readjusted, for example, by moving the objective system in an axial direction. To maintain the focus on a selected information layer, the focus error detection system with the composite diffraction element and the two pairs of detection elements as described in U.S. Pat. No. 4,665,310 can be used.
When, in a record carrier with two information layers, the information layer located furthest remote from the objective system is being read, radiation is not only reflected by this layer but also by the information layer located closest to the objective system. In contrast, when the information layer located closest to the objective system is being read, radiation will not only be reflected by this layer but also by the layer located furthest remote from the objective system because the first-mentioned layer passes radiation to the last-mentioned layer which reflects the radiation. As a result, not only a said detection spot is formed on each detector pair, but also an additional, or extra, spot. Such an additional spot has an asymmetrical radiation distribution with respect to the separating strip of the relevant pair of detection elements, because the radiation originates from an information layer which is situated out of focus, so that the output signals from the detection elements of this pair are not equal, even if the scanning beam is focused on the selected information layer. Thus, an offset is produced in the focus error signal. Since the focus servo will correct in such a way that the focus error becomes zero, the scanning beam is then no longer satisfactorily focused on the selected layer.
Those skilled in the art are further directed to review U.S. Pat. Nos. 4,924,079, 4,908,506 and 4,829,506. The above citations are hereby incorporated in whole by reference.
It is an object of the present invention to provide a device of the type described in the opening paragraph in which this problem is prevented. To this end, the device according to the invention is characterized in that the surface of the composite diffraction element consists of a first area in which a main part of the first grating is located, a second area in which a main part of the second grating is located, and a third, elongated transition, area in which further parts of the first and the second grating are located, the surface occupied by the first grating in the transition area being, in principle, equal to the surface occupied by the second grating.
The invention is based on the recognition that by choosing, instead of a straight boundary line between the two gratings, a transition area in which equal parts of both gratings are located, an additional radiation spot can be given such a shape that the detection elements of a pair associated with that spot receive the same amount of radiation from the additional radiation spot so that this spot cannot cause any difference between the output signals of the elements of this pair. Thus, the cause of a focus offset is eliminated in a simple manner. In this context, xe2x80x9cin principle, equalxe2x80x9d is understood to mean that the surfaces of the first and the second grating within the transition area are equal if the scanning beam has a uniform intensity, whereas these surfaces may slightly differ, dependent on the intensity distribution, if the scanning beam does not have a uniform intensity, for example, a Gaussian intensity distribution.
A preferred embodiment of the device according to the invention is further characterized in that the transition area consists of a first and a second portion in which a part of the first grating and a part of the second grating, respectively, are located.
An alternative embodiment of the device is further characterized in that the transition area has a boundary line, on one side of which a part of the first grating is enclosed by two parts of the second grating, and on the other side of which a part of the second grating is enclosed by two parts of the first grating.
Within the transition area, the gratings may be further subdivided so that more than three parts of the first grating and of the second grating are located in this area.
In principle, it is sufficient when the detection system comprises a pair of detection elements which co-operates with one of the detection spots, and a third detection element which co-operates with the other detection spot. The difference between the output signals of the detection elements of the pair then comprises information about a possible focus error, while the output signal of the third detection element, in combination with the output signals of the pair of detection elements, represents the signal which has been read and/or comprises information about a possible tracking error.
A preferred embodiment of the device is, however, characterized in that the detection system comprises a first and a second pair of detection elements for the first and the second detection spot.
By suitable combination of the output signals of the four detection elements, a focus error signal can be obtained which has a better signal-to-noise ratio and, moreover, is insensitive to an alignment error of the boundary line between the gratings with respect to the detection system, respectively.
This embodiment is preferably further characterized in that the separating strip in each pair of detection elements is oriented in such a way that:
the path along which the center of the intensity distribution of the associated detection spot is displaced at a change of the wavelength of the scanning beam coincides with said separating strip, and
said separating strip is, in principle, parallel to a line connecting the center of the emissive surface of the radiation source to the center of the radiation-sensitive detection system.
In this embodiment, a wavelength change, which may occur when using a diode laser as a radiation source, has no influence on the focus error signal, while the device can still be assembled easily.
As regards the structures of the two gratings and the associated arrangement of the pairs of detection elements, the device according to the invention may be implemented in various ways. These possibilities are defined in claims 7-9.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.